1. Field of the Invention
The present invention concerns a procedure for removing/reducing the directionality or anisotropy of a surface and making the surface specular on a substrate material consisting of aluminium or an aluminium alloy.
The substrate material can be produced by casting, extrusion, rolling or other shaping techniques and the surface properties of the substrate material may be of greater or lesser significance. For example, stringent requirements have to be made for the surface properties of a substrate material which is to be used for reflector purposes. Materials which are to reflect visible light are particularly surface-critical and the description which follows will concern such materials in particular.
In order for a material to be suitable as a reflector material, such as for lighting purposes, it must meet a number of requirements. The material must have high reflectivity for light and it should preferably have certain light dispersion properties, depending on the application. Moreover, the material must have sufficient scratch resistance to allow cleaning. Any layers on the surface of the material must adhere well to the substrate. The material should also be as light as possible and may typically be based on rolled aluminium.
2. Description of the Related Art
Already as early as in the thirties, it was realised that aluminium meets many of the above requirements. The reflectivity of this material is exceeded only by silver. The shaping properties of aluminium are good and its weight is low. For functional and aesthetic reasons, it used to be desirable that lighting materials had as bright a surface as possible. Chemical and electrochemical polishing processes were developed which made it possible to transform the relatively rough rolled surface of a highly pure material into a bright surface. In addition, these processes contributed to increasing the reflectivity of the rolled substrate. Aluminium is a soft material and when it comes into physical contact with other materials, damage occurs, for example in the form of scratches. In addition, a bare aluminium surface is exposed to corrosion. For most applications of aluminium, including for lighting purposes, it is, therefore, necessary to protect the surface. The most common methods of protection are anodisation or lacquering.
After 1945, aluminium started to be used on a large scale for lighting purposes. The production of reflectors in aluminium was a typical piece part process. First the reflector parts were shaped to form the desired geometry and then they were assembled to form a complete reflector or screen. The screens were then polished chemically or electrochemically and subsequently protected by anodisation. Lacquering was also used as a treatment after the electrochemical process.
A common feature of the piece part or batch processes was the long treatment time required to polish the relatively rough starting material. With the chemical processes then in use, the quality of the starting material was decisive if the treatment time was to be reduced. Gradually, rolling mills improved their processes and became able to supply materials with considerably greater brightness. Thus it becomes possible to reduce the polishing times, which gradually allowed for the introduction of continuous brightening process for aluminium strips.
Continuous plants for chemical or electrochemical polishing and anodisation were taken into use in the mid-fifties. Continuous surface treatment was considerably more cost-effective than the traditional piece part processes. Rolling mills, particularly in Europe, continued to improve their processes and the need for polishing become less and less. Today, the brightness of the material is mainly created in the rolling mill. Lines for electrochemical polishing and anodisation have become less important as a consequence. They are mainly used to remove a surface layer in order to increase the reflectivity of the material. The brightness is usually not altered significantly as a consequence of this. Perhaps the most important task of the brightening line today is thus to form protection for the aluminium by means of the anodisation which normally follows.
Specular materials are produced today by cold-rolling pure aluminium alloys (typically 99.8% Al or purer, for example AA 1080) in a rolling mill. The material""s reflectivity is subsequently increased by means of chemical or electrochemical treatment, which involves removing a thin layer of the material""s surface without altering the brightness significantly. The material is then protected by anodisation or lacquering. The requirements for the rolling quality and alloy quality (among other things, its purity) are usually regarded as being fundamental for a satisfactory result.
The surface is polished either chemically or electrochemically. The electrochemical polishing process is generally preferred when polishing a bright starting material and has the potential to produce a more specular surface than chemical polishing. Chemical polishing, in turn, can be used to advantage to polish raw materials with a somewhat rougher rolling quality. Chemical polishing is today regarded as the best process for giving relatively rough surfaces a considerable increase in brightness. However, as rolling mills now usually supply very bright material, chemcial polishing is gradually becoming a less relevant process. If the starting material is already bright, the electrochemical process will produce a brighter end product than the chemcial process.
Both chemical and electrochemical polishing may result in topographical disturbances or so-called gassing defects. In today""s production processes, it is desirable to avoid these defects. This is done by using a starting material of a sufficient quality and by correcting the process conditions in the process bath.
The problems associated with gassing defects are discussed and described in the following article:
xe2x80x9cThe incidence and avoidance of gassing defects in chemical and electropolishing of aluminium A. W. Clifford and D. J. Arrowsmith, University of Aston in Birmingham, Dept. of Metallurgy and Materials, Feb. 23 1978xe2x80x9d.
The tests described in this article were carried out in acid mixtures which are difficult to regenerate. Particularly in processes which involve major removal of substrate material, it will be of great significance that the chemical can be regenerated expediently.
In order to obtain a satisfactory result with the processes described in the article, the treatment time will easily be as much as 10 minutes. With treatment times of this order, these processes will not be feasible in connection with continuous treatment of substrate materials (strip process).
Furthermore, aluminium materials with a low content of alloy elements are preferably used as, according to the article, the alloy elements (in particular the size of the intermetallic particles) are assumed to be of major significance in relation to the formation of defects on the surface of the substrate. The article supports the general view that a sufficient purity of the aluminium material is necessary in order to be able to control and prevent the development of defects so as to achieve a satisfactory end product.
The present invention concerns a procedure which makes it possible to produce a material with qualitatively good optical properties based on aluminium alloys of semi-bright rolling quality, which is a considerably rougher quality than that which is currently used for the production of specular lighting materials. Moreover, in accordance with the present invention, it is possible to use a less pure, less expensive starting material than previously, for example qualities such as AA 1200 (Al less than 99.2%) or AA 1050 (Al less than 99.5%) for the production of specular material. Moreover, it is possible to use recycled Al metal, for example AA 1200 recycled, which results in environmental gains. Moreover, the process uses chemicals which can be regenerated, which results in both environmental and financial gains. In accordance with the present invention, this has been made possible by means of a new, improved electrochemical polishing process. In accordance with the present invention, the brightness may be increased more than by means of processes of the prior art on the basis of the specified raw materials and available treatment times. This applies both to the relative increase in brightness between the starting material and the finished material and to the absolute brightness in the finished product. With the improved process in accordance with the present invention, the treatment time can also be reduced in relation to processes of the prior art, which makes it possible for the process to be used in a plant for the continuous treatment of a substrate material.